Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale.

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This discussion session interrogated the current understanding of hydrogen embrittlement mechanisms in steels.This article is part of the themed issue 'The challenges of hydrogen and metals'.

The effect of nitrogen additions on fatigue behavior has been examined in near-equiaxed, rolled Ti-6Al-4V bar. This is the first-time nitrogen content that has been systematically explored with respect to monotonic and cyclic properties in a Ti-6Al-4V alloy base composition. Nitrogen additions were found to increase the $$\beta $$ β -transus temperature and strength, but they decreased ductility, even in microstructures where some $$\beta $$ β phase remained. This carried across into both the low- and high cycle fatigue behavior; even small contents of 240 and 560 ppmwN caused reductions in both low cycle fatigue life and high cycle fatigue strength. In samples containing 240 and 560 ppmwN, a conventional striated fractographic appearance was observed, but a dramatic change to a macroscopically brittle fracture surface was observed at 1800 and 3600 ppmwN, but still with substantial evidence of plasticity at the microscale. Therefore, neither microstructure or fractographic examination, nor EDX-based compositional analysis in the electron microscope are necessarily a reliable indicator of an absence of deleterious nitrogen contamination. This is significant for the investigation of potentially nitrogen-contaminated surface-initiated cracks, either due to service or processing exposures.

Advances in aero engine technology require constant innovation in development and in-service management of key engineering materials. Improvements to alloy performance and component management relating to fatigue cracking depend on mechanistic understanding of damage accumulation processes at the atomic scale, such as the influence of alloying elements on slip behaviour in α+ titanium alloys. Many of these materials contain sufficient aluminium for thermomechanical processing to induce nanometre-scale precipitation of the crystallographically ordered α 2 (Ti 3 Al) phase. This phase impedes dislocation motion, causing intragranular slip heterogeneity that leads to high micromechanical stresses and ultimately increases the propensity for fatigue crack initiation [1].